If S knows OW and has spent some time with her before, and if H was gone like 1-2 hours, then I don't think I would make an issue out of it. There's a difference between him getting a babysitter to watch S for an evening out vs. needing to run an errand and just having her make sure he doesn't burn the place down for a few minutes. It sounds like it was closer to this, so the idea of him looping you in doesn't seem like it was necessary. As far as how well S knows OW, that I can't speak to, but I'd assume that it's not the first time they've met if H made this decision, and as you say it doesn't sound like there was any real danger.
For all of those reasons I think it's not a hill to die on. I wouldn't chide him on the secrecy either because he probably thought it was a non-event but one that you might overreact on, and in some ways I can see where he's coming from (although he should know S would tell you). I'd say don't react the way he'd expect you to, letting this slide goes a long way in demonstrating that you trust him as a parent, and that goes a long way in cementing a good co-parenting R, and that is very important for your S.
Nothing fun about knowing that WAH is modeling dating other women prior to D, but to me that's nothing compared to modeling walking out on a marriage and destroying a family in the first place. Either way it's one of those things we can't do anything about. All we can do is just model the behavior and values WE believe in and letting our children have that experience to draw on as they grow.
All in all I think you vent here, curse D, WAS's, OP that get involved, and how hard it is...then you smile, shrug it off, lead on for your family, and focus back on how awesome your life is.
Take care and Merry Christmas!
Me:38 XW:38 T:11 years M:8 years Kids: S14, D11, D7 BD/Move out day: 6/17/14, D final Dec 15
